The present invention relates to a method of inspecting articles for defects and the like. The invention is particularly useful in automatic optical inspection systems for printed circuit boards (PCBs), and is therefore described below with respect to this application.
Printed circuit boards are often optically inspected with respect to a reference, namely a flawless or almost flawless board. Such a reference is characterized by a list of features created either by processing a board known to be substantially flawless, or by processing computer aided manufacturing (CAM) data of such a board. The list of features may include any number of features, typically from a few thousand to tens of thousands. Each feature is identified by its coordinates (x,y), and by a code indicating the feature type and enabling the indexing of as many different types of features as necessary for any given PCB.
To find defects in a PCB that is to be inspected, a list of features characterizing the inspected PCB (commonly called an online list) is prepared and is then compared with a reference list of the features previously prepared and stored for comparison purposes. In order to produce the online list, the PCB is optically scanned, and its image is processed to detect the features therein. Any feature appearing in the online list with no match in the reference list is termed an "excess feature" and is reported as a defect. Similarly, any feature appearing in the reference list with no match in the online list is termed a "missing feature" and is also reported as a defect.
When the online list and the reference list are in perfect geometrical registration, the matching is quite simple since it is only necessary to compare the two lists and to determine whether the feature type and coordinates of one match those of the other. In reality, the coordinates are never in perfect geometrical registation so that such a simple comparison procedure cannot be performed. Therefore, the coordinates are not required to be exactly the same, but they should be close enough, as specified by the user, to allow a discrepancy parameter, termed "reference tolerance", to be used in the matching. The reference tolerance is typically considerably smaller than the design grid of the PCB. If it is smaller than the manufacturing process precision or the feature-detector precision of the optical inspection system, the false alarm rate of the inspection will increase.
It will thus be appreciated that the comparison of the inspected article (sample) with the reference is greatly facilitated by having the sample placed on the scanner with as perfect registration as possible to the reference. Also important is the geometrical accuracy of the inspection system itself.
Various mechanical, optical or electronic aids are presently used for registering PCBs in order to permit their optical inspection with respect to a reference list of features.
Mechanical aids usually take the form of registration pins, sideboards, and the like; see for example U.S. Pat. Nos. 4,783,826 and 4,799,175. Using such mechanical means requires special and different preparations for different types of PCBs. For example, using registration pins requires constructing the pins on the scanner table, punching holes in the articles to be inspected, and manual or other mechanical alignment of the holes with the registration pins. Such mechanical aids thus involve significant additional costs and time for this extra mechanical handling.
Computational registration is often achieved by having easily detected fiducial marks in perfect registration with the PCB pattern; see for example European Patent Application 0206712. This method requires an extra scan for the fiducials prior to inspection, and therefore significantly increases the overall inspection time. Only after the fiducials have been scanned can the misregistration be calculated and fixed. Also, the fiducial marks require allocations of extra space on the PCBs in addition to the functional PCB pattern.
Some systems, for example as disclosed in U.S. Pat. No. 4,805,123, carry out local registration by pixel-wise correlating reference image to online image. This requires the reference images to be stored, instead of just the characteristic features. Because of the extreme computational complexity of carrying out correlations between images, such a system can only compensate for smaller amounts of misregistration and can be implemented, as a practical matter, only on special-purpose hardware.
Still other systems, such as described in European Patent Application 0247308, employ dynamic distortion control to compensate for local distortions, rather than for global misregistration. Such systems, therefore, still require preliminary registration, for example according to one of the above-described techniques.